Crystal controlled frequency modulation system



Jan. 11, 1949. 2,458,760

CRYSTAL CONTROLLED FREQUENCY MODULATION SYSTEM' W. M. A. ANDERSEN FiledAug 17, 1945 IQQWSQ xhx v n Patented Jan. 11, 1949 CRYSTAL CONTROLLEDFREQUENCY MODULATION SYSTEM Walther M. A, Andersen, Hartford, Conn.,assignor to Crystal Research Laboratories, Incorporated, Hartford, Conn,a corporation of Connecticut Application August 17, 1945, Serial No.611,186

4 Claims.

My invention relates broadly to modulation systems, and moreparticularlyto an improved circuit arrangement for a crystal controlledfrequency modulation system.

One of the objects of my invention is to provide an improved circuitarrangement for a frequency modulation circuit which is extremely simplein construction and which has excellent fidelity of modulation.

Another object of my invention is to provide a construction of frequencymodulation system in which a minimum number of tubes are utilized forproducing frequency modulations under control of amplitude modulations.

Another object of my invention is to provide a circuit arrangement formodulating crystal controlled oscillator systems for developingoscillations which are impressed upon a mixer circuit for deriving afrequency modulated difference frequency with respect to the frequenciesof the crystal controlled oscillator systems.

Still another object of my invention is to provide a circuit arrangmentfor amplitude modulating a twin three electrode electron tube forproducing frequency modulations of extremely high fidelity. I

Other and further objects of my invention reside in a circuitarrangement for a frequency modulation system as set forth more fully inthe specification hereinafter following in which Fig. 1 schematicallyillustrates a circuit arrangement embodying the modulation system of myinvention, and Fig. 2 shows an application of a modified form of thecircuit arrangement embodying my invention.

My invention is directed to acircuit arrangement for obtaining frequencymodulation with a simple mezzo-electric crystal controlled circuit. Theadvantages of my circuit are its extreme simplicity and the excellentfidelity of modulation. I provide a piezoelectric crystal controlledcircuit in which an audio modulating voltage of small magnitude isarranged to vary the bias potential applied to the crystal controlledcircuit in such manner that frequency deviation of the crystal iscontrolled With corresponding high fidelity of modulation of thecircuit. In one of the applications of the system of my invention twocrystal controlled oscillator circuits of the type hereinbeforedescribed are combined to produce wide band frequency modulation. Twopiezoelectric crystals of different frequencies are modulated inopposite directions by the push-pull audio input circuit of an amplitudemodulated system. The frequency deviation is doubled in the output of anassociated mixer circuit. The adjustment of cathode bias potentiometersin the circuits of the two crystal controlled oscillators will determinethe frequency deviation and the fidelity of modulation. Such modulationmay vary with different tubes and piezoelectric crystals, and thecircuits of my invention provide compensation for all such variations.

Referring to the drawings in detail reference character i, Fig. 1,discloses a three electrode electron tube including a heater la, acathode lb, a control grid lo, and an anode id. The grid lc and cathodeto are connectedthrough a high impedance circuit designated as 2, asincluding a resistor 3 and shunt connected condenser 4. The resistor andcondenser combination are con nected into the cathode circuit throughthe bias resistor 5 shunted by condenser 6. The bias resistor 5 has anadjustable tap l movable thereover for rendering a selected portionthereof ineffective for controlling the bias potential supplied to thecontrol grid lc. Suitable means are provided for activating cathode la.The anode Id is energized from a suitable potential source connected toterminals 8 through radio frequency choke coil 9. A piezoelectriccrystal In is connected across control grid to and plate Id. An outputcircuit II is connected between plate Id and cathode lb, comprising asshown, the capacity l2 and resistor [4. An audio frequency controlcircuit I5 is connected across a portion of the high impedance circuit,accomplished by providing an adjustable tap E6 on high resistance 3 andconnecting the adjustable tap to one side of the. audio frequencycontrol circuit, where the other side of the audio frequency controlcircuit connects to the end of the resistance 3 as represented at H.

Oscillations at the frequency of piezo-electric crystal ID are sustainedin the electron tube circuit including the high impedance path 2. Theseoscillations are modulated by the audio frequency variations impressedthrough circuit I5 across a portion of high impedance circuit 2 whichvaries the bias potential on control grid lo and ole-- velops frequencymodulations in the output circuit ll. Output circuit it connects toharmonic multipliers and power amplifiers of a frequency modulationsystem.

The effective energy developed by the piezoelectric crystal lfl deviatesin frequency as the load varies in the high impedance circuit 2, thusthe audio frequency variations impressed through circuit I5 directlycontrol the deviation in frequency of piezoelectric crystal I0 anddevelops frequency modulations in the output circuit II.

In Fig. 2 I have shown a system in which a high degree of precision isobtained in wide band frequency modulation. In this arrangement I employa twin three electrode electron tube represented at I8, containing twosets of electrodes indicated at I9 and 26. The set of electrodes I9 isconstituted by heater 19a, cathode I91), control grid I90, and anode |9darranged in the order shown. The set of electrodes 20 includes heater20a, cathode 2%, control grid 29c, and anode 20d. Suitable means areprovided for activating cathodes |9a and 20a. The anodes |9d and 26d areenergized from a suitable potential source connected to terminals 2|through radio frequency choke coils 22 and 23 respectively.

The input circuits for each of the twin sets of electrodes are both ofhigh impedance represented at 24 and 25. The input circuit 24 includeshigh resistance 26 shunted by condenser 27 and connected to the controlgrid I90 and cathode |9b through the bias control circuit 28. The biascontrol circuit 28 includes high resistance 29 shunted by condenser 36.The effective value of resistance 29 is controlled by movement of anadjustable tap 3| along resistance 29, thereby controlling the potentialapplied to control grid I90.

The high impedance input circuit 25 consists of high resistance 32shunted by condenser 33 and connected at one end to control grid 26c andat the other end to cathode 20b through the bias control circuit 34,which contains high resistance 35 shunted by condenser 36. An adjustabletap 3'! on resistance 35 controls the effective value of resistance 35for correspondingly controlling the bias potential on control grid 200.

Two piezo-electric crystals 38 and 39 are employed in the oscillationsystem of my invention. Piezo-electric crystal 38 connectsbetween'control grid I90 and anode I9d. Piezo-electric crystal 39connects between control grid 20c and anode 26d. These crystals are ofdifferent frequencies, displaced from each other in the frequencyspectrum to provide for a beat frequency therebetween. The crystalfrequencies are modulated in opposite directions by a push-pull audiofrequency input. I have illustrated the modulation circuit as includingan audio frequency input at transformer 40, having a primary winding 4|connected to the audio frequency modulation circult, and a secondarywinding 42 coupled thereto. The secondary winding 42 has a central tap43 connected to an intermediate point 44 between the cathodes I91) and2% through the bias control circuits thereof. The opposite ends ofsecondary winding 42 are connected to adjustable taps 45 and 46 whichare adjustable along high resistances 26 and 32. Thus,'a portion of thehigh resistances 26 and 32 in the high impedance circuits 24 and 25 arerendered ineffective by short circuit between the ends which connectwith the intermediate point 44 between the cathode circuits and thepositions of the adjustable taps 45 and 46.

Circuits 24 and 25 are aperiodic in character and oscillations aresustained through the electron discharge paths across the respectivesets of electrodes I9 and 20, at frequencies corresponding to thefrequencies of piezo-electric crystals 38 and 39. These oscillations areimpressed upon a mixer circuit which consists of pentagrid tube 41having a heater 41a, a cathode 411), a pair of control grids 41c and41d, an output grid 416, a space charge grid 41 and a plate 419. Theanodes of the oscillator tube I8 at I? and 20d are respectivelyconnected to control grids 41b and 41c through coupling condensers 48and 49. The cathode 41b is connected through bias control circuit 50with the opposite end of the output circuit of the oscillator tube I8.

The mixer tube 41 has a tank circuit 5| connected across plate 41g andthe output grid 41c thereof through impedance path 52 as shown. The tankcircuit 5| is disposed in the output circuit of tube 41 in circuit withhigh potential source connected with terminals 53. The output grid 41chas condensers 54 and 55 connected in circuit therewith on oppositesides of high impedance '52 and leading to the cathode side of the mixertube. An output circuit is connected to the circuit 5| as represented bycondenser 56 and high impedance 5'! from which connections lead toharmonic multipliers and power amplifiers of a frequency modulationsystem.

In the circuit of Fig. 2 audio frequency variations impressed upon theprimary winding 4| change the effective bias potential on control grids|9b and 2922, thereby controlling the effective energy developed by thepiezo-electric crystals 38 and 39 and the impression thereof upon themixer circuit 41. The output frequencies will deviate as the sum of thedeviation of the two crystal oscillators. The frequency deviation isdouble in the output circuit 56-5'| of the mixer circuit of tube 41, thetwo frequencies from piezo crystals 33 and 39 producing a beat frequencyin the electronic path in tube 41 deriving a stabilized frequencymodulated signal in the output system 55-5'|.

I have found the circuits of my invention highly effective in operationand while I have disclosed my invention in certain preferred embodimentsI realize that modifications may be made and I intend no limitationsupon my invention other than may be imposed by the scope of the appendedclaims.

What I claim and desire to secure by Letters Patent of the United Statesis:

1. A frequency modulation system comprising a twin three electrode tube,including sets of cathode grid and plate elements, means for activating.said cathodes, means for energizing said plates, a high impedance pathconnected between the grid and cathode of each of said sets of elements,a piezo-electric crystal connected between the grid and plate of each ofsaid sets of elements, said piezo electric crystals being displaced infrequency for producing a beat frequency therebetween, a tap in each ofthe high impedance paths associated with said grid and cathode elements,an audio frequency control circuit connected between said taps and to apoint intermediate said cathode elements and a mixer circuit comprisinga pentagrid tube including a cathode, a pair of control grids, an outputgrid, an anode and a space charge grid, a tank circuit tuned to thedifference frequency between the aforesaid piezoelectric crystals andconnected between said output grid and said anode'and connectionsbetween said plate elements of said twin three electrode tube and therespective control grids.

2. A frequency modulation system comprising a twin three electrode tube,including sets of cathode grid and plate elements, means for activatingsaid cathodes, means for energizing said plates;

a high impedance path connected between the grid and cathode of each ofsaid sets of elements, a piezo-electric crystal connected between thegrid and plate of each of said sets of elements, said piezo electriccrystals being displaced in frequency for producing a beat frequencytherebetween, a tap in each of the high impedance paths associated withsaid grid and cathode elements, an audio frequency control circuitconnected between said taps and to a point intermediate said cathodeelements and a mixer circuit comprising a pentagrid tube including acathode, a pair of control grids, an output grid, an anode and a spacecharge grid, a tank circuit tuned to the difference frequency betweenthe aforesaid piezo-electric crystals and connected between said outputgrid and said anode connections between said plate elements of said twinthree electrode tube and the respective control grids, and a condenserdisposed in series with each of said last mentioned connections andoperating as capacity coupling members.

3. A frequency modulation system comprising a twin three electrode tubeincluding sets of cathode grid and plate elements, means for activatingsaid cathodes, means for energizing said plates, a high impedance pathconnected between the grid and cathode of each of said sets of elements,a piezo-electric crystal connected between the grid and plate of each ofsaid sets of elements, said piezo electric crystals being displaced infrequency for producing a beat frequency therebetween, an adjustable tapin each of the high impedance paths associated with said grid andcathode ele ments, an audio frequency control adjustable circuitconnected between said taps and to a point intermediate said cathodeelements, and a mixer circuit comprising a pentagrid tube including acathode, a pair of control grids, an output grid, an anode and a space:charge grid, a tank circuit tuned to the difference frequency betweenthe aforesaid piezo-electric crystals and connected between said outputgrid and said anode and connections between said plate elements of saidtwin three electrode tube and the respective control grids 4. Afrequency modulation system comprising a twin three electrode tubeincluding sets of cathode grid and plate elements, means for activatingsaid cathodes, means for energizing said plates, 3, high impedance pathconnected between the grid and cathode of each of said sets of elements,a piezo-electric crystal connected between the grid and plate of each ofsaid sets of elements, said piezo-elec'tric crystals being displaced infrequency for producing a beat frequency therebetween, a tap in each ofthe high impedance paths associated with said grid and cathode elements,an audio frequency control circuit connected between said taps and to apoint intermediate said cathode elements, adjustable means for controlling the bias potential on each of said grid elements and a mixercircuit comprising a pentagrid tube including a cathode, a pair ofcontrol grids, an output grid and an anode, a tank circuit connectedbetween said output grid and said anode, connections between the plateelements of said twin three electrode tube and the respective controlgrids of said pentagrid tube and means in said tank circuit foradjustably tuning said tank circuit to the diiferenc'e frequency betweenthe frequency of each of said piezo-electric crystals.

W'ALTHER M. A. ANDERSEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 40 2,309,083 Usselman Jan. 26,1943 2,390,777 Cole Dec. 11, 1945

